In an individual section glass container forming machine, a gob of molten glass is dropped into a blank mold and formed by a plunger into a parison. Such plungers have a long stroke, most of which is from a fully retracted or down position, to a partially up position where the top of the plunger will support the gob when it is dropped into the blank mold. Parison formation results from the continued upward displacement of the plunger to its fully up position.
U.S. Pat. No. 4,274,859 discloses tandem concentric pistons for controlling the displacement of the plunger. Both pistons are displaced conjointly for rapid displacement and the inner piston, which is connected to the plunger, moves separately when slow displacement is to occur. As a result, different cylindrical surfaces guide the plunger during rapid and slow displacement and this is undesirable.